Question

Given two subquery tables t1 and t2, how can I return t2 if and only if t1 returned empty rows?

Edit: Added example

T1

SELECT * 
  FROM common_table 
 WHERE language_id = 1

T2

SELECT * 
  FROM common_table 
 WHERE language_id = 2

Basically what I am doing is that in case the T1 return empty rows, I would like it to execute T2 and return those rows. Now, I am fully aware that I can do this in PHP but the query is a subquery and I would rather let SQL (not PHP) code handle it.

Answer 1

In MSSQL I can do something like

If EXISTS (SELECT * FROM common_table WHERE language_id = 1)
BEGIN
    SELECT * FROM common_table WHERE language_id = 1
END ELSE BEGIN
    SELECT * FROM common_table WHERE language_id = 2
END

Should be the same in mySQL or similar